Create list of generics

Question

I have base class for my entities

public class Entity<T> where T : Entity<T>, new()
{
    public XElement ToXElement()
    {
    }
    public static T FromXElement(XElement x)
    {
    }
}

I have to use this strange construction Entity<T> where T : Entity<T>, because i want static method FromXElement to be strongly-typed Also, i have some entities, like that

public class Category : Entity<Category>
{
}
public class Collection : Entity<Collection>
{
}

How can i create a generic list of my entities, using base class?

var list = new List<Entity<?>>();
list.Add(new Category());
list.Add(new Collection());

Answer 1

You can't with that definition. There is no "common base class" between Category and Collection (other than object, of course).

If there were, say if Entity<T> were defined as:

public class Entity
{
}

public class Entity<T> : Entity where T : Entity<T>, new()
{
    public XElement ToXElement()
    {
    }
    public static T FromXElement(XElement x)
    {
    }
}

then you could do

var list = new List<Entity>();
list.Add(new Category());
list.Add(new Collection());

But what would that buy you?

Answer 2

Create a marker interface:

public interface IAmAGenericEntity { }

public class Entity<T> where T : IAmAGenericEntity, new()
// ...

public class Category : Entity<T>, IAmAGenericEntity
// ....

var list = new List<IAmAGenericEntity>();
// ...